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In computing (particularly, in programming), undefined value is a condition where an expression does not have a correct value, although it is syntactically correct. An undefined value must not be confused with empty string, boolean "false" or other "empty" (but defined) values. Depending on circumstances, evaluation to an undefined value may lead to exception or undefined behaviour, but in some programming languages undefined values can occur during a normal, predictable course of program execution. Dynamically typed languages usually treat undefined values explicitly when possible. For instance, Perl has undef operator which can "assign" such value to a variable. In other type systems an undefined value can mean an unknown, unpredictable value, or merely a program failure on attempt of its evaluation. Nullable types offer an intermediate approach; see below.== Examples == The value of a partial function is undefined when its argument is out of domain of definition. This include numerous arithmetical cases such as division by zero, square root or logarithm of a negative number etc.; see NaN. Even some mathematically well-defined expressions like exp(100000) may be undefined in floating point arithmetic because the result is so large it cannot be represented. If an implementation supports ±∞, then this value may be computed as +∞ (inf), however. An element of an array is undefined when index is out of bounds, as is a lookup in an associative array for a key which it does not contain. An argument of variadic function, which was not passed to it, is undefined within the function body. A variable which is not initialized, has undefined (or unpredictable) value until it was assigned. Dereferences of null pointers lead to undefined value and usually raise an exception immediately. Any expression of the bottom type is undefined by definition, because that type has no values. The value of a function which loops forever (for example, in the case of failed μ operator in a partial recursive function) may be seen as undefined too, but only of a theoretical interest because such function ''never returns''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Undefined value」の詳細全文を読む スポンサード リンク
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